1. Technical Field
The invention relates to the use of scanned light beams to detect the presence and location of an object(s), stationary or otherwise, in a defined field. Once the object's presence and location are determined, the coordinate data generated may, for example, be correlated to a numerical value and displayed. Alternatively, if the object is a pen-like object, an image may be drawn on a corresponding display screen by moving the object through the field.
2. Description of the Related Art
The use of light beams to detect the presence and location of moving or stationary objects in a field is well documented. For example, U.S. Pat. Nos. 3,807,858 and 4,097,800 disclose the use of light beams for detecting the passage of a projectile through a field. French Patent No. 2,630,222 discloses the use of a plurality of light beams reflected off a rotating mirror to scan a monitored area and detect the presence of a fast moving object through the field.
U.S. Pat. No. 4,762,990 discloses an integrated light timing and rotational arc displacement system to establish the coordinates of the location of an object in a work area by the angular displacement of a single scanning light beam from a starting position until the beam is interrupted by the separate serially occurring events of intersection of a virtual image beam and the actual beam. The invention employs a directly reflecting peripheral member, opposite to the location of the scanning light beam. A retroreflecting light enhancing peripheral member may be employed on portions of the remaining periphery to detect a variety of styli or omitted to detect a retroreflecting stylus. The prior art, however, fails to overcome many problems associated with accurately detecting the presence and location of an object in a field and does not provide a simple and economical design.
For example, the system disclosed in U.S. Pat. No. 4,762,990 is overly complex and inaccurate. Actual and virtual beams are swept across the field and the beams are at all times diverging. This is problematic when detecting the presence of multiple objects in the field because shadows from other objects in the field are created by the diverging beams, which results in inaccurate readings. Even the presence of small objects in the field of such a system can create large shadows, which in turn cause serious inaccuracies in the position readings.
In addition, U.S. Pat. No. 4,762,990 contemplates the use of a retroreflector on the periphery of the field to maintain the background light level. Since the light beam is swept across the field, the angle of incidence of the light beam upon the retroreflector is constantly changing. The varying angle of incidence on the retroreflector results in a variable reflective efficiency and therefore the reflective response is not constant with time. This variance negatively affects the overall retroreflective efficiency and hence the accuracy of the system.
An additional problem associated with a system such as that disclosed in U.S. Pat. No. 4,762,990 is the effect of the corner on the illumination of the field. The reflective corner results in a region across the field which cannot be illuminated and therefore will result in a null in the signal. Consequently, there exists a region in which the system cannot measure objects accurately.
In addition to the above problems, scanning systems such as those described in U.S. Pat. No. 4,762,990 require the means of reflection to be intimate with and adjacent to the field being scanned. Specifically, the system illustrated in U.S. Pat. No. 4,762,990 does not permit the means of reflection to be remote from the field being scanned. Thus, design and aesthetic considerations are limited by the requirement that the reflective optics be intimate with and adjacent to the defined field.